So I had plans to ride my R90S today. Had, because the electrical system was completely unresponsive this morning. Battery is OK so I started to trouble shoot. Check connections, looks for the obvious. Nothing. Eventually I get to pulling the fairing to poke around inside and behind the headlight. This bike has been modified with a Dyna electric ignition so its not stock. I found this connector inside the head lamp. It has a red fuse inside. The fuse has completely disintegrated and there is corrosion on one end. Obviously I need to replace this thing. Anyone recognize the fuse and can recommend a source for parts?

So I have figured out the fuse is a Bosch 16a torpedo or continental type fuse on a wire from the diode board to the starter relay. It was not blown, just heavily corroded.

None of the wiring diagrams for that time period show a fused wire between the diode board and the starter relay. I don't know of any BMW motorcycle that would have had that. Wonder what the PO was trying to "fix" or hide. Also, the Dyna III has no connections inside the headlight shell so it's not associated with that.

Yes it is a 16A fuse in a non-stock inline connector. I removed and directly connected the wire. Bike functions as normal now. Fires right up and I took a brief test ride. No issues. I'll soon have to dig in to see why a previous owner may have made this modification. I don't have much in the way of service history. The connector is definitely a recent addition as there is almost zero evidence of age on one side. The other, however looks like it had experienced some significant heat which leaves something to be concerned about. I don't like the idea of wires generating heat.

So the saga continues. Bike ran fine for a few weeks. The other day it just quit at a stoplight. Dead as a doornail. Same issue. No power going anywhere. I walked the bike out of traffic and started to poke around inside the headlight.

It appears that this particular red wire and wiggled off the circuit board. Reconnected but still nothing, so something else may have also happened. Still have to chase wires but I did discover yet another previous owner "modification". The two fuses that should be in the bucket on the board have been relocated to up near the handlebars. I'll have to see how they have rewired but I suspect that all of these mods over the last 40 years are leading to these electrical gremlins.

I'll post pics as I tear thing apart but has anyone ever seen something where those fuses were relocated?

I had a similar problem after I restored my R90S. The bike would run fine then it would just stop. I beat myself up over it for weeks then I decided to go back into the headlight. I double checked all of the connections to verify everything worked then I put rubber caps on all of the "open" exposed terminals. My thought process at the time was ..... when I pushed all the wired back into the headlight to reinstall the lens something always seemed to short out. Call me crazy but all the issues stopped. I have not had a short or an electrical issue since.

The saga continues. The bike is still unresponsive electrically. The battery is fine, Just confirmed, battery tight, tried using an external battery. Still no dice. Voltage is OK at all of the places I can think to check. Everything appears connected in the headlight bucket, right handlebar switch appears to check out, ignition switch tests out, starter relay is actually quite new looking. I’m stumped.

To catch up the bike (unfortunately) has seen a few “modifications” over the past 40+ years. Dual plugs, DYNA ignition, fuses remoted up to the instrument panel area. Clock has not worked for years but otherwise the machine is a clean high mile (89K) example of a DO R90S.

So I was continuing the poking around and with the key in the on position, kill switch in run still no electricity evident (lights, clicks, etc. nothing.). Still have voltage in the headlight bucket where appropriate.

Curious thing I just discovered. The two red wires feeding the starter relay under the tank show 12 volts when the key is off but drop to zero when the key is on. Seems backwards to me.